Engine systems may be equipped with exhaust gas recirculation systems that recirculate a portion of exhaust gas from an engine exhaust to an engine intake system to improve fuel economy and reduce regulated emissions. The recirculated exhaust gas may dilute the oxygen concentration of the intake air resulting in reduced combustion temperatures, and consequently, formation of nitrogen oxides in the exhaust may be reduced. In order to achieve improved engine operation and reduced emissions, a targeted EGR flow rate and air/fuel ratio may be maintained by adjusting engine actuators.
An EGR valve may be included in an exhaust gas recirculation path to control an amount of exhaust gas recirculated to achieve a desired air intake dilution. Turbocharged engine systems may include a low-pressure EGR (LP EGR) system which recirculates exhaust gas from the exhaust passage downstream of a turbine to the intake passage upstream of a turbocharger compressor. Accordingly, exhaust gas may be recirculated into a low-pressure air induction system (LP AIS) upstream of the compressor, resulting in a compressed mixture of fresh intake air and EGR downstream of the compressor.
EGR flow may be measured based on differential pressure across the EGR valve and a flow area determined by EGR valve lift. Errors in EGR flow measurement may lead to reduced fuel economy, decreased engine performance and increased emission. Therefore, EGR flow must be monitored and controlled during engine operation to provide optimal EGR, and air/fuel mixture for combustion. One example approach for controlling EGR flow rate is illustrated by Song et al. in US 2012/0073179 A1. In Song et al., a first EGR flow rate is estimated based on oxygen concentration at the intake and a second EGR flow rate is estimated based on engine speed and engine load. Based on differences between the two flow rates, a fault in the EGR system is indicated.
However, the inventors herein have identified issues with such systems. For example, in determining EGR flow, Song does not take into account changes in valve lift or valve area which may occur due to soot build up at the EGR valve. Non-uniform soot build-up may lead to changes in effective valve area, which may result in EGR measurement errors. Consequently, delays and insufficiencies in EGR flow may occur, which in turn may result in degraded fuel economy, engine performance and emission.
Therefore, in one example, some of the above issues may be at least partially addressed by a method for an engine, comprising: closing an exhaust gas recirculation (EGR) valve; reducing an intake throttle opening until a differential pressure across the closed EGR valve reaches a threshold; and while the differential pressure is maintained, learning an EGR leakage flow correction based on intake oxygen and adjusting the EGR valve during open EGR valve operation based on operating parameters and the EGR leakage flow correction.
In another example, a method for an engine may comprise: reducing an opening of an intake throttle while closing an EGR valve; and estimating an EGR valve leak area based on an intake manifold oxygen sensor located upstream of the throttle. The method may further comprise correcting an EGR flow rate based on the estimated EGR valve leak area.
In still another example, a method for an engine may comprise reducing an opening of an intake throttle while adjusting the lift of an EGR valve; and learning the transfer function of EGR flow versus valve lift based on an intake manifold oxygen sensor located in the diluted air flow stream. The method may further comprise correcting an EGR flow rate based on the learned transfer function.
In this way, methods for an engine control system may be provided to accurately learn the changes in EGR valve lift and adjust the flow rate based on the valve lift. By learning the effective leaking area of the EGR valve during valve-closed conditions, and/or by learning the differential pressure transfer function during different valve lift conditions, adjustments to the effective EGR valve area may be performed. As a result, more accurate EGR flow measurements may be obtained and consequently, improved fuel economy, improved engine performance and reduced emissions may be obtained. Further, a change in EGR flow from a nominal flow may be calculated by utilizing the learned leak area and transfer function. Upon determining that the change in EGR flow is greater than a threshold change, EGR valve degradation may be indicated. By determining EGR valve degradation based on the learned leak area and/or transfer function, more accurate leak diagnostics may be obtained and more accurate control of EGR may be achieved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.